In automotive vehicles and other systems in which a drive such as a prime mover, e.g. an internal-combustion engine, is coupled to a load, e.g. the driven wheels of the vehicle, through a gear-change or speed change transmission, it is common to provide a clutch between the output shaft (crankshaft) of the prime mover and the input shaft of the transmission.
The transmission can be of a fluid-dependent or fluid operated type in which the member, e.g. a rod, link or lever controlling the speed ratio between the input and output shafts of the transmission, is ineffective in the absence of a fluid pressure supplied to the transmission. This control member can operate, for example, a valve which controls the flow of compressed air to actuators of clutches or the like within the transmission to effect gear change, speed change or ratio change.
It is a common practice in such systems as well to provide a clutch pedal which is operated by the driver of the vehicle and which is effective through a clutch-operating cylinder to actuate the clutch and decouple the output shaft of the invention from the input shaft of the transmission.
The transmission can be directly or indirectly actuated by the compressed air. A direct actuation is effected when the transmission-control member operates a valve in the manner described. An indirect actuation may use the compressed air to block movement of the control member which can be mechanically connected to the shifting mechanism in the transmission.
In automotive vehicles and especially construction equipment, yard vehicles, earth-moving equipment and the like in which the transmission is generally heavily loaded, it is essential to prevent shifting of the transmission while the clutch is engaged. Thus devices and mechanisms have been provided heretofore to prevent shifting of the transmission while the clutch is engaged, such devices being connected with the clutch-actuating rod. One problem with such devices is that, even upon operation of the clutch pedal by the driver of the vehicle, complete decoupling cannot be guaranteed before the gear change is initiated in the transmission.